1. Field of the Invention
The present invention relates to a heat-sensitive recording paper and a process for production thereof.
2. Development of the Invention
Recently, heat-sensitive recording methods have been widely utilized in the field of printing such as output in computers, desk computers, etc., the field of recorders for medical analyses, the field of low speed and high speed facsimiles, the field of automatic ticket vending machines, the field of heat-sensitive recording paper, etc. because of (1) non-impact and no noise in recording, (2) no need for development and fixing, (3) easy maintenance of machines, etc.
Microcapsules have heretofore been often utilized in heat-sensitive recording paper. For example, Japanese Patent KOKOKU No. 70/74 discloses multi-color heat-sensitive recording paper obtained by coating a mixture of two or more chromogenic substances (dye precursors) which form colors at different reaction temperatures, an electron acceptor (color developer) and a water-insoluble substance capable of dissolving both dye precursor and color developer (which substance is microencapsuled in the case of a liquid) on a paper sheet and drying the coated paper sheet.
Japanese Patent KOKOKU No. 15227/74 discloses a heat-sensitive recording paper obtained by coating onto the surface of a paper sheet a dispersion of capsules having enclosed therein an electron donor (dye precursor), capsules having enclosed therein a color former of ionic reaction system and an electron acceptor (color developer) in wax.
Japanese Patent KOKAI No. 12695/82 discloses a heat-sensitive recording paper obtained by coating onto the surface of paper microcapsules obtained using a color former (dye precursor), a color developer and microcapsules of a sublimating, heat-fusible substance compatible with at least either the dye precursor or the color developer. After pressing the recording paper (destroying microcapsules) and then recording by heat, recorded images are fixed.
In the foregoing techniques, however, substances which dissolve dye precursors or color developers are microencapsuled and all involve drawback in storability that microcapsules are destroyed by the pressure in non-use as in so-called pressure-sensitive recording paper to cause contamination on the coated surface.
On the other hand, Japanese Patent KOKAI No. 145046/77 discloses a heat-sensitive recording paper in which both of a chromogenic substance (dye precursor) and an electron acceptor (color developer) or any one of these two components are microencapsuled in a finely divided state, which microcapsules are prepared by a complex coacervation method using a polyanion comprising phytic acid or metal salts thereof. However, this technique is microencapsulation limited to a complex coacervation method using phytic acid and metal salts thereof, and is not general. Further, although it is a fact well known to one skilled in the art, the technique involves a drawback that when the heat-sensitive recording paper is attempted to directly perform recording by heat in a state that microcapsules are not destroyed, the microcapsules are not destroyed upon the recording by heat so that color forming reaction scarecely occurs.
On the other hand, as heat-sensitive recording is widely utilized over various fields, high sensitivity and production in a large scale are desired. However, requirement for high sensitivity is contradictory to requirement for production in a large scale. This is because as high sensitivity is desired, coating should be carefully performed at a low speed and a low temperature in order to prevent occurrence of fog at the background. In spite, coating at a high speed and a high temperature is required for realizing production in a large scale.